Explaining recent trends in extreme precipitation in the Southwestern Alps by changes in atmospheric influences

We analyze recent trends in extreme daily precipitation in the Southwestern Alps. We consider a high-resolution precipitation dataset of 1 x1 km² for the period 1958-2017. A robust method of trend estimation is considered, based on nonstationary extreme value distribution and a homogeneous neighborhood approach. The results show contrasting trends in extreme daily precipitation depending on the season. In autumn, the trends are significantly increasing in most of the Southwestern Greater Alpine Region, with an increase up to 100% the average maxima for the 20-year return level between 1958 and 2017, while the French Alps show mainly decreasing extremes. Knowing that autumn experiences most of the largest maxima, the increase in the Mediterranean area is of concern for risk protection.  In winter, the valleys and medium mountain areas surrounding the Northern French Alps show significant increasing extremes, while the inner French Alps, the Swiss Valais and the Aosta Valley show significant decreasing trends. In the other seasons, the significant trends are mostly negative in the Mediterranean area, while the French Alps show less organized and contrasting trends.  For all seasons, part of the significant changes in extremes can be related to changes in the dominant atmospheric influences generating the maxima, particularly in the Mediterranean influenced region that shows the most organized trends. In particular, the strong positive trends in autumn in Southern France are concomitant with an increase in Mediterranean influence generating the maxima. However some exceptions are notable with counter-intuitive trends in extremes given the trends in dominant influences.